(1) Field of the Invention
The present invention relates to a calibration apparatus and method for an array antenna.
(2) Description of the Related Art
Recent communication systems, represented by CDMA (Code Division Multiple Access), require larger capacity and higher speed, and an array antenna is employed as a means to realize these requirements. That is, the CDMA system is based on an access mode in which channels are allocated through the use of codes to carry out simultaneous communications, while interference with signals from other channels under the simultaneous communication condition occurs, which consequently limits the number of simultaneously communicable channels, i.e., the channel capacity. An array antenna, particularly an adaptive array antenna, is employed for the purpose of enhancing this channel capacity.
The adaptive array antenna adaptively forms a beam according to an environment for a user, who wants to do, and forms a null to a user who becomes a source of large interference, and is a technique capable of increasing the channel capacity. That is, it forms a beam in a direction of the user who wants to do and directs a null toward the user who becomes a source of large interference so as to receive radio waves with high sensitivity from the user who wants to do, but never to receive radio waves from the large interference source. This can reduce the amount of interference, which results in an increase in channel capacity.
Meanwhile, the adaptive array antenna is designed to produce a beam through the use of a phase difference at antenna terminal portions. For this reason, when a phase variation occurs at radio portions, proper control of beam pattern becomes impossible. Accordingly, the proper control of the beam pattern requires a correction of the phase difference at antenna terminal portions, and the calibration between antenna elements is of the essence.
FIG. 10 is an illustration for explaining a calibration method for a reception system of an array antenna, and corresponds to FIG. 1 of Japanese Patent Laid-Open No. 2004-297694. An array antenna system (reception system) as shown in FIG. 10 is comprised of an array antenna unit 100 including a plurality of (four) antenna elements Ant1, Ant2, Ant3 and Ant4, switches 101 each provided for each of the antenna elements Ant1, Ant2, Ant3 and Ant4, array radio receivers (RXs) 102, timing detectors 104, multipliers 106 and 108, an adder 110, an adaptive controller 112, a reference signal memory 114 and a calibration signal memory 116 serving as a radio transmitter dedicated to a calibration signal.
In this array antenna system, calibration signals read out from the calibration signal memory 116 are inputted through the array radio receivers 102 and the timing detectors 104 to the multipliers 106 and the adaptive controller 112. The multipliers 106 weights the output signals from the timing detector 104 with correction weight coefficients W01, W02, W03 and W04. The adaptive controller 112 calculates the aforesaid correction weight coefficients W01, W02, W03 and W04 on the basis of output signals from the timing detectors 104, a reference signal from the reference signal memory 114 and output signals from the multipliers 106 according to an adaptive algorithm based on the least-square error method. Thus, the array antenna system itself can carry out the calibration, which reduces the system scale at the calibration.
However, the above-described calibration method requires the radio transmitter 116 dedicated to a calibration signal for the calibration of a reception system, and requires a radio receiver dedicated to a calibration signal for the calibration of a transmission system. In addition, since the calibrations of the transmission system and the reception system are made separately, a transmitter-receiver dedicated to a calibration signal becomes necessary for both the transmission and reception calibrations. This dedicated transmitter-receiver is a redundant radio apparatus which is not used in actual operations, and the presence of this radio apparatus causes an enlargement in apparatus scale and circuit scale and entails an increase in dissipation power and cost.